Purification of cellulose material



Patented Aug. 22, 1933 UNITED STATES PATENT OFFICE No Drawing.Application August 31,1931 Serial No. 560,521

6 Claims.

This invention relates to a process of purifying cellulosic material andmore particularly to a pretreatment of cellulosic material containingcoloring matter to facilitate a more complete removal of the coloringmatter by a subsequent digesting process, such as an alkali boil-out.

As is well known, certain cellulose base materials such as kraft pulpcontains a large proportion of coloring matter so that such product is10 adapted for use as a base material only when the products to beprepared therefrom are satisfactofy if colored, .such as wrapping paperand the like. However, in so far as I am aware, no commerciallypractical process has been developed for treating highly coloredcellulose material so as to render it suitable for making light coloredor white products. The processes which have been produced either havebeen too expensive for commercial utilization or have not been efflcientin removing the color from the material treated. The principal purposeof the present invention is to provide a commercially practical processfor removing coloring matter from highly colored cellulose material.

As indicated above, the present process is particularly designed for usein connection with the removal of color from kraft pulp. However, it issuitable for use in, removing coloring matter from any other form ofpulp andis also suitable for treating cotton linters containing coloringmatter and for removing coloring matter from screenings from the kraftprocess. tice of the process the material to be treated is intimatelycontacted with a weak mineral acid solution. The intimate contact of thematerial to be treated with the weak acid solution may be effected bythorough agitation of the materials together, or, as is preferable, bysubjecting the material to the action of a boiling weak mineral acidsolution. The strength of the acid solution employed and the time oftreatment will vary according to the material worked upon; the

time will also vary according to the temperature of the treatingsolution. For example, a longer 4-5 period of time will be required whenintimate contact is effected by agitation of the materials in the coldthan in the case where the material is subjected to a boiling treatingsolution. After intimately contacting the material with the weak acidsolution the acid is removed, the product washed and subjected to aconventional digesting treatment, such as an alkali boil-out. Thedigesting treatment removes the major amount of coloring matter presentand a subsequent wash ing of the material with hot water whitens it Inthe 'prac- I appreciably. To whiten the material completely it isadvisable to follow the hot water wash with a light chlorine bleach.

In the preferred practice of the process the cellulosic material. to betreated is subjected to Go the action of a weak nitric acid solution ashereinafter described in detail; However, I may employ weak solutions ofother mineral acids in place of the nitric acid solution. For example,

a hydrochloric acid solution of from .035 to 2.0 5 per cent. strengthmaybe employed. Where kraft pulp is to be treated the hydrochloric acidsolution is preferably of .075 per cent. strength. A sulphuric acidsolution of from .015 to 2.0 per cent. strength may also be optionallyemployed, a 79 solution of about .03 per cent. strength being preferredwhen kraft pulp is being treated. However, the use of a sulphuric acidsolution is not recommended because of its marked tendency to lowerviscosity. Phosphoric acid solutions of from .025 to 5 per cent. arealso suitable, a solution of about .075 per cent. strength beingsuggested for usewhen kraft pulp is to be treated. Moreover, sulphurousacid solutions of from .015 to 2 per cent. strength may be used in thepraca0 tice of the invention, a solution of .05 per cent. strength beingpreferred for treating kraft pulp. In addition to the foregoing otherweak mineral acid solutions which are suitable will suggest themselvesto those skilled in the art.

While it seems most expedient to prepare the weak solutions of mineralacids by adding the acids directly to water'and. the material undertreatment, I have found that substantially the same, and in some casesslightly better, results can be obtained by adding a mineral salt and aweak organic acid and forming the mineral acid in situ. For example, afinal product of superior fiber strength is obtained by treating thecellulosic material with a weak nitric acid solution formed by reactingsodium nitrate and acetic acid-in the treating bath. Among other mineralsalts which may be usd'in place of sodium nitrate in conjunction withacetic acid are sodium chloride, sodium phosphate, potassium sulphate,sodium bisulphite, or the like. These are typical of the salts which maybe used. While sodium and potassium salts are preferred because of thesolubility of their compounds, sim ilar salts of other metals may beemployed in cases where the precipitation of a mineral on the cellulosicmaterial is not harmful. Besides acetic acid other weak organic'acidsmay be employed. Among such acids which are suitable no be found toproduce best-results.

I effect on theflber. that after the acid solution has once been colareoxalic, tartaric, and citric acids and fatty acids such as oleic,stearic and palmitic acids.

As stated above, I prefer to employ in the practice of my process a weaknitric acid solution and, according to the material under treatment, Ihave found it advisable to employ a nitric acid solution of from .05 to5 per cent. strength. In treating linters containing coloring matter a.05 to 1 per cent. nitric acid solution is satisfactory. For treatingkraft pulp from .05 to 2 per cent. nitric acid solution will Where kraftscreenings are to be treated a stronger solution, say from 4 to 5 percent. nitric acid solution, is

recommended. The time of treatment also varies, as stated above,according to the material under treatment. For example, when a coldsolution is used, linters are agitated in the acid solution for aboutone hour only. For kraft pulp, an agitation of 3 hours is recommended.If hot solutions are used, it is sufficient to bring the mass to theboiling point only with linters, while for kraft pulp, hour at boilingis recommended. Kraft screenings should be boiled for an hour and, insome instances, boiling for a longer period will be found desirable.

The first use of a fresh acid solution tends to make the fibers tenderbut I have found that after the acid solution has once been used in theprocess it does not thereafter have any adverse Moreover, it has beennoted ored by use in the process the color does not deepen when the acidsolution is reused. Moreover, there does not appear to be anysubstantial loss of acid, except for mechanical losses, in the practiceof the process. Thus, these last two features indicate that the aciddoes not react chemically with the coloring matter so as to render itmore easily removable by a subsequentdigest. However, regardless of whatmay be the real action of the acid on the coloring matter present it hasbeen definitely ascertained that the coloring matter is more readily andwashed material is then subjected to a conventional hot alkalitrfltment, either an open boiling or a digestitng opreation beingsuitable. Generally, the treatment consists of boiling the material inan 8 to 10 per cent. caustic soda solution for about one hour in adigester. Following the alkali treatment the material is preferablysubjected to the washing action-of an excess of hot water and it may befurther subjected to the action ofa light bleach as set forth above.

The term rendering active as applied to the weak mineral acid solutionin the appended claims is intended to include both the substantialagitation of the acid solution in'contact with the cellulosic materialas described above in connection with one embodiment of the inventionand also heating the acid solution to boiling condition as disclosed inconnection with the preferred embodiment of the invention.

7 By the term weak solution of a mineral acid". as employed in theappended claims is meant 9.

mineral acid solution having a strength not materially in excess of 5per cent., the particuar strength of a given acid solution dependingupon the particular mineral acid present. For example, a nitric acidsolution of from .05 to 5.0 per cent. strength, a hydrochloric acidsolution of from .035 to 2.0 per cent. strength, a sulphuric acidsolution of from .015 to 2 .0 per cent. strength, a phosphoric acidsolution of from .025 to 5.0 per cent. strength and a sulphurous acidsolution of from .015 to 2 per cent. strength are considered weakmineral acid solutions in connection with the practice of the presentinvention and are intended to be included within the meaning of the termin question.

While I have described in detail the preferred embodiment of myinvention it is to be understood that the details of procedure may bevariously modified without departing from the spirit of the invention orthe scope of the subjoined claims.

I claim:

1. The process of treating cellulosic material containing coloringmatter to be removed therefrom which comprises contacting such materialwith a solution of nitric acid of from .05 to 5 per cent. strength,rendering such solution active with respect to said cellulosic material,removing the acid, washing the thus treated material, thereaftersubjecting the thus pretreated material to a hot alkali purifyingtreatment, washing the resulting product with hot water, and subjectingthe resulting product to the action of a bleaching agent.

2. The process of removing coloring matter from kraft pulp whichcomprises contacting such kraft pulp for a period of up to approximately3 hours with a nitric acid solution of about .05

to 2 per cent. strength rendered active with respect thereto, removingthe acid, washing the thus treated material, subjecting the washedproduct to a purifying hot alkali treatment, washing the resultingproduct with hot water, and subjecting the thus washed product to alight bleach.

3. The process of removing coloring matter from kraft pulp whichcomprises contacting such pulp for a period of up to approximately 3hours with a nitric acid solution of from .05 to 2 per cent. strengthrendered active with respect thereto, pressing out the excess of acid,washing the thus treated material, digesting the washed material forapproximately 1 hour in a caustic soda solution of from approximately 8to 10 per cent. based on dry weight of pulp, washing the digestedmaterial with hot water, and subjecting the thuswashed product to alight bleach of approximately 8 per cent. chlorine strength, based ondry weight of pulp.

4. The process of treating cellulosic material containing coloringmatterto be removed therefrom which comprises subjecting such material to apretreatment including contacting such ma terial with a solution ofnitric-acid of frem .05 to 5 per cent. strength, rendering such solutionactive with respect to said cellulosic material, removing the acidsolution, and washing the thus treated material, and thereaftersubjecting the pretreated material to the action of a bleaching agent.

5. The process. of treating cellulosic n; containing coloring matter tobe removed from which comprises subjecting such material to apretreatment including contacting the material with a solution of nitricacid of from .05

treating steps including subjecting such kratt pulp to the action of asolution of nitric acid of approximately .05 to 2 per cent. strengthrendered active with respect thereto, removing the acid solution, andwashing the resulting material, and thereafter subjecting the pretreatedmaterial to the action of a bleaching agent.

HARRY P. BASSETI.

